Selective Single Molecule SERRS of Cationic and Anionic Dyes by Cl– and Mg2+ Adions: An Old New Idea

We show in this study the selective single molecule surface-enhanced resonance Raman scattering (SM-SERRS) detection of cationic Rhodamine 6G (R6G) and anionic Rose Bengal (RB) dyes on colloidal silver nanoparticles (AgNPs), promoted by Cl– and Mg2+ adsorbed ions (adions), respectively. From a mixture of R6G and RB, we detected the SM-SERRS spectrum of R6G on AgNPs@Cl–, whereas on AgNPs@Mg2+ we observed the surface-enhanced Raman scattering (SERS) spectrum of a single RB molecule. From a total of 6000 SERS spectra acquired (3000 on AgNPs@Cl– and 3000 on AgNPs@Mg2+), 57 were SM-SERS events, having a statistical distribution characteristic of SM-SERS, as determined by the modified-PCA statistical method. Out of the 57 SM-SERS events, only 15 were mixed events of both R6G and RB, and no cross events were observed (i.e., no RB event on AgNPs@Cl– or R6G event on AgNPs@Mg2+), showing a remarkable selectivity. To the best of our knowledge, this is the first report showing such a spectacular SERS selectivity at the single molecule level, solely by using Cl– (for cationic molecules) or Mg2+ adions (for anionic molecules). Finally, we show and discuss general trends governing the adsorption of anionic and cationic molecules on colloidal AgNPs, and we suggest possible mechanisms (surface adsorption sites formed by adions, surface Hoffmeister effect) which can help us understand the role of adions in SERS experiments and in surface science in general.